This invention generally relates to Raman lasers, and is particularly concerned with a multi-spectral line Raman laser.
The many advantages associated with Raman optical amplifiers has resulted in their increased use in optical telecommunication networks. Such amplifiers are based on stimulated Raman scattering, known in the art as SRS effect. In such amplifiers, a source of pump light is coupled to a length of gain fiber. The pump photons scatter off the vibrational modes of the gain fiber""s lattice matrix and coherently add to the lower-energy (longer wavelength) signal photons. Unlike erbium-doped fiber amplifiers, the gain fiber used in Raman amplifiers may be the transmission fiber itself. Additionally, since the SRS effect is not dependent upon the energy level structure of a dopant, gain may be provided at essentially any wavelength if a corresponding source of pump light is available.
Despite these advantages, the practical use of Raman amplifiers has been limited by the very high pump light power that is typically required to achieve usable gain (caused by the fact that the SRS effect is a nonlinear optical effect), and the relatively narrow spectral output of such amplifiers which, of course, limits its signal transmission spectrum.
The problem of high pump power has been largely solved by the development of cascade Raman lasers having output powers of greater than one watt. In a standard configuration, the cascaded Raman laser is pumped by a high-power, double-clad Nd or Yb fiber laser and generates light in the 1420-1490 nm spectral range. This light can then be launched backwards in the fiber transmission line, which functions as the gain fiber, and provides distributed Raman amplification of the optical signal conducted through the line in the last 20 kilometers or so of the fiber span. In many current system designs, such Raman amplification is combined with the amplification from erbium-doped fiber amplifiers (EDFAs) in order to capitalize on the advantages of both, i.e., the lowered effective noise figure of a distributed Raman amplifier, and the high efficiency of an EDFA.
Unfortunately, in such cascaded Raman lasers, narrow spectral output is still a problem as only one relatively narrow spectral line is generated which corresponds to the fifth or sixth Stokes order of the Yb or Nd fiber laser pump. By contrast, at least two spectral lines with approximately 25 nm spacing (typically at about 1429 and 1455 nm) are needed to achieve a relatively flat gain curve, i.e., gain which does not change with wavelength by more than about 1 decibel in the C-band spectral range which corresponds to between about 1535-1565 nm. At least three different spectral lines are needed if flat amplification is desired for a combined C and L band amplifier, which covers a spectral range of between about 1535-1610 nm. Even if the narrow spectrum problem associated with such cascaded Raman lasers is solved by the generation of two or more spectral lines, some sort of means of dynamically adjusting the relative amounts of power generated between the spectral lines is necessary in order to compensate for the possible xe2x80x9ctiltxe2x80x9d or xe2x80x9cripplexe2x80x9d which might occur between the powers of the channels transmitted by the amplifier.
One technique for generating multiple spectral line outputs in Raman lasers is to split the Yb or Nd fiber laser output into several approximately equal parts and use them to pump several cascaded Raman wavelength converters, each with its own spectral output line. However, because threshold power conditions need to be met in every converter, the overall efficiency in such a technique is limited. Additionally, the amount of components needed to achieve multiple spectral lines is effectively doubled or tripled, thereby increasing package size, complexity, and cost.
Another approach employs two pairs of tunable fiber Bragg gratings as reflectors in a cascaded Raman resonator to generate a power output at two different spectral lines. By simultaneously stretching the two pairs of gratings, a controlled wavelength mismatch may be created between back and front reflector spectral profiles, thereby changing the amount of optical feedback and hence the power provided to the two lasing lines. However, the mismatching of the reflectors also results in degraded efficiency, since a same percentage of the resonating light can escape out of the back of the lasing cavity as is generated out of the front end.
In a variation of the aforementioned approach, variable reflection from a temperature-tuned FBG is employed. Relatively long output FBGs are coated with varying thickness electric resistance heaters. Uneven heating causes xe2x80x9cchirpxe2x80x9d in the FBG, decreasing maximum reflection. Simultaneously, the maximum reflection wavelength is shifted, which necessitates the use of a second uniform thickness electrical resistance heater which uniformly stretches the FBG to compensate for the shift. While this particular technique provides both multiple spectral lines and a means for adjusting the amount of relative power present in each line, it also requires the adjustment of two separate parameters to change each output mirror reflection, i.e., an adjustment of both the non-uniform and the uniform electrical resistance heaters to create the xe2x80x9cchirpxe2x80x9d in the FBG while simultaneously compensating for the shift in maximum reflection wavelengths. Such dual adjustment of the heaters is not only cumbersome, but slow. Finally, because this technique depends upon the introduction of chirp in the grating period, the width of the resulting reflection bands may change, which in turn can cause unwanted mismatches between the back and front reflectors and thereby degrade lasing efficiency.
Clearly, what is needed is a multiple-spectral line Raman laser having a means for dynamically controlling the relative power output of the different spectral outlines which is simple and inexpensive in structure and operation. Ideally, such a Raman laser would be capable of generating at least three spectral lines in its power output so that the resulting gain will be relatively flat. Finally, the mechanism for dynamically controlling the relative power between the spectral lines should not result in light losses which substantially degrade lasing efficiency.
Generally speaking, the invention is a multiple spectral line Raman laser that overcomes or at least ameliorates the aforementioned shortcomings associated with the prior art. To this end, the laser of the invention comprises a lasing cavity, first and second reflectors optically coupled to a back end of the cavity that reflect substantially all light having wavelengths of xcex1 and xcex2, respectively, and a tunable reflector assembly optically coupled to a front end of the cavity that reflects a selected proportion of light having wavelengths xcex1 and xcex2 in response to a single source of strain in order to control the relative power output of light at these wavelengths.
In one embodiment of the invention, the tunable reflector assembly includes a stretchable fiber Bragg grating (FBG) which has a trapezoidal reflection profile. The amount of reflection of xcex2 changes substantially linearly as strain is applied to the optical fiber that the FBG is written on, and the reflection band is at least 4 nm in width. Additionally, the wavelengths xcex1 and xcex2 are separated by at least 4 nm and at most by 30 nm. The single source of strain includes a mechanism, such as a piezoelectric driver, that stretches or relaxes the FBG.
In another embodiment, the tunable reflector assembly includes a pair of fiber Bragg gratings (FBGs) mounted on opposite sides of a flexible substrate. A single source of strain applies a bending force on the substrate to stretch one of the FBGs while simultaneously compressing the other. Like the first embodiment, the single source of strain may include a piezoelectric driver.
In both the aforementioned first and second embodiments, the lasing cavity is linear, and additional rear reflectors and tunable reflector assemblies may be optically coupled to the back and front ends of the cavity, respectively, in order to reflect selected portions of light having a wavelength between xcex3 and xcex4, etc.
In still a third embodiment of the invention, the lasing cavity is a fiber ring cavity and the tunable reflector assembly includes a wavelength-selective fiber coupler that couples the front and back of the ring cavity to input and output waveguides, respectively. First and second reflectors are coupled to the input waveguide, and third and fourth reflectors are coupled to the output waveguide. The wavelength-selective fiber coupler transmits variable proportions of light having wavelengths of xcex1 and xcex2, in response to an application of a mechanical bending force. The single source of strain of the tunable reflector assembly in this embodiment may include a stepping motor that extends and retracts a driving rod via a gear train.